1. Field of the Invention
The present invention relates to a quasi-coherent detection in which a quadrature detection of modulated input signals at a local oscillator frequency is made, and a phase compensation is made on baseband signals converted from the detected input signals.
2. Description of the Background Art
Conventionally, a quasi-coherent detection, in which the quadrature detection of the modulated input signals at a local oscillator frequency is made and the phase compensation is made on the baseband signals converted from the detected input signals, has been developed in the field of low speed data transmission. However, in recent years, due to the increasing speed of the digital circuits, there are cases in which the quasi-coherent detection is utilized for the high speed data transmission.
An example of a conventional quasi-coherent detection circuit for the high speed data transmission is disclosed in Ohtani et al. "Development of Digital MODEM LSI for Satellite Communications", Electronic Information Communication Society, Satellite Communication Study Session SAT88-7, pp.39-45, which has a configuration shown in FIG. 1. This configuration of FIG. 1 is basically a conventionally known carrier recovery circuit configuration implemented in terms of baseband range digital signals. In FIG. 1, a signal line for a complex quantity is indicated by a thick line, while a signal line for a scalar quantity is indicated by a thin line.
More specifically, in this circuit of FIG. 1, a carrier range complex multiplier 1 makes the quadrature detection of the modulated input signals S at a local frequency of a local oscillator 2, and after the noise components are removed by low pass filters 3, the detected input signals are converted into digital signals at A/D converters 4 and entered into a digital PLL (Phase Lock Loop) circuit 9.
At the digital PLL circuit 9, the phase rotation due to the frequency error is corrected at a digital complex multiplier 5. Then, the phase error is detected at a phase comparator 6, and the error voltage is integrated at a loop filter 7, so as to provide the feedback from a digital VCO (Voltage Controlled Oscillator) 8 to the digital complex multiplier 5.
Thus, in this quasi-coherent detection circuit, the acquisition time is determined by the time constant and the loop gain of the loop filter 7 which are the loop parameters of the digital PLL circuit 9.
In this configuration of FIG. 1, even though the high speed acquisition characteristic can be achieved by enhancing the noise band width, the quality of the demodulated signals can be deteriorated by such an enhancement of the noise band width.
Another example of a conventional quasi-coherent detection for the high speed data transmission is a block demodulation for the burst signals disclosed in Namiki, "Block Demodulation for Short Radio Packet", Electronic Information Communication Society Proceedings, "84/1, Vol. J67-B, No. 1, pp. 54-61, which can be implemented by a circuit configuration shown in FIGS. 2 and 3.
More specifically, in this circuit shown in FIG. 2, the quasi-coherent detection of modulated input signals S having N symbols for forming one burst is made by the carrier range complex multiplier 1, the local oscillator 2, the low pass filters 3, and the A/D converters 4, which are similar to those in the circuit of FIG. 1 described above.
Then, the carrier recovery is achieved by the averaging process using the power multiplication operations and the least square algorithm, where the total error including the initial phase error and the frequency error is estimated by using the least square algorithm, and the phase rotation is corrected according to the result of this estimation at a phase rotation correction estimation unit 10 shown in detail in FIG. 3.
Namely, in the phase rotation correction estimation unit 10 shown in FIG. 3, when the modulated input signals S are the M-array phase shift keying modulation signals, the tentatively demodulated signals U obtained at the A/D converters 4 are temporarily stored in a memory unit 11 shown in FIG. 2. Meanwhile, the modulation components are removed by multiplying the tentatively demodulated signals U for M times at an M-th power multiplication circuit 12-1. Then, the phase rotation is corrected at a digital complex multiplier 5-2 by calculating a complex multiplication of the beat components obtained by the M-th power multiplication circuit 12-1 and the correction signals for up to the immediately previous symbol multiplied for M times by another M-th power multiplication circuit 12-2. Then, the least square estimation value for the immediately previous symbol is corrected by the newly obtained estimation value at an estimation value correction circuit 13. Then, the estimation value for the n-th symbol is converted at a time series complex correction generation circuit 14 into the complex signals Z=exp[-j{.theta..sub.0 +.delta..omega.(n-1)T}] with the initial phase difference equal to .theta..sub.0 and the frequency error equal to .delta..omega.(n-1)T, where .delta..omega. is the frequency error and T is a symbol period.
Next, the tentatively demodulated signals U including the beat components which are stored in a memory circuit 11 are multiplied with the complex signals Z obtained by the phase rotation correction estimation unit 10 sequentially at a digital complex multiplier 5-1 to obtain the final demodulated signals.
However, this block demodulation requires a memory circuit with a memory capacity equal to the length of the burst, as well as many complex multipliers and adders, so that the circuit size inevitably becomes quite large.
In addition, a delay of at least one burst length is required in obtaining the final demodulated signals, so that it is not applicable to an application requiring the real time operation.
Another example of a conventional quasi-coherent detection for the high speed data transmission is a method for estimating the phase of the carrier by using the linear estimation, disclosed in Sampei, "QPSK Coherent Detection Method for Land Mobile Radio Communications Using Linear Mean Square Estimation", Electronic Information Communication Society B-II, Vol. J 72-B-II, No. 4, pp. 125-132.
However, even though this method can make the configuration of the circuit simpler, a steady state phase error occurs in a case involving a frequency error, so that the proper demodulation cannot be achieved in a case involving a frequency error in this method.